Method of detecting a redirection or relaying of a contactless data transmission using at least two sequentially driven transmitting antennas

ABSTRACT

A method detects relaying of a contactless data communication by a radio amplifier for intercepting and manipulating the communication to achieve an unauthorized authentication. The method involves transmitting an electromagnetic field successively from at least two spatially separated antennas of a transmitting unit, receiving the transmitted fields in a receiving unit, determining field strength vectors of the received fields allocated to each antenna, and comparing actual values of the vectors with prescribed nominal values stored in a nominal value field that maps the true field strength distribution in multi-dimensional space around the transmitting unit. The actual field strength magnitude and the reception angle between the transmitting antennas can be determined from the vectors and compared with stored nominal values in this manner. When the determined actual values match prescribed nominal values of the nominal value field, the occurrence of a relaying of the signal can be reliably excluded.

PRIORITY CLAIM

This application is based on and claims the priority under 35 U.S.C.§119 of German Patent Application 101 59 604.9, filed on Dec. 5, 2001,the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for detecting or recognizing theredirection or relaying of a contactless data transmission, especiallysuch a redirection or relaying that is unauthorized.

BACKGROUND INFORMATION

A method of the above mentioned general type is known from the GermanPatent Laying-Open Document DE 101 06 736 and corresponding U.S. PatentApplication Publication 2002-0109581-A1, wherein it is determinedwhether a redirection or relaying of a contactless data transmission hastaken place, by comparing the damping characteristics of thetransmission link of the “uplink” and the “downlink” between a firsttransmitting and receiving unit and a second transmitting and receivingunit. For this purpose, the amplitude value of the transmittedinterrogation signal is measured in the second transmitting andreceiving unit, and then this value is transmitted back to the firsttransmitting and receiving unit. Then, in the first transmitting andreceiving unit, the value of the amplitude of the interrogation signalthat has been transmitted back is compared with the value of theamplitude of the response signal that has been received by the firsttransmitting and receiving unit. An unacceptable deviation between thesetwo amplitude values in the result of this comparison is an indicationthat a redirection or relaying of the data transmission has taken place.In actual practice, however, it is a disadvantage that this method doesnot make it sufficiently difficult to carry out a redirection orrelaying using a so-called transceiver, i.e. it remains relatively easyto achieve an undetectable redirection or relaying using such atransceiver.

A further method as background to the present invention is known fromthe German Patent Laying-Open Document DE 100 05 503 and correspondingU.S. Patent Application Publication 2001-0014117-A 1, wherein at leastone physical signal value or characteristic parameter of the transmittedelectromagnetic wave is reversibly altered. To achieve this, a reply orresponse signal is altered relative to the interrogation signal, forexample with respect to the frequency, in a second transmitting andreceiving unit (i.e. a transponder), and is transmitted back to thefirst transmitting and receiving unit (i.e. a base station). In thefirst transmitting and receiving unit (i.e. the base station), thefrequency of the response signal is altered or changed back again andcompared with the frequency of the originally transmitted interrogationsignal. If the frequency value determined in this manner lies within aprescribed interval, then it can be essentially completely excluded thata redirection or relaying has occurred.

Yet another method of the general type forming the background of thepresent invention is known from the German Patent Laying-OpenPublication 100 19 277, in which a redirection or relaying is detectedby means of a comparison of the transit time of the interrogation andresponse signals with prescribed values. In other words, a redirectionor relaying is positively detected when the measured value of thetransit time does not correspond with a prescribed value of the transittime.

Systems for the contactless transmission of data are preferably used inidentification systems. Generally, such systems comprise a firsttransmitting and receiving unit, i.e. a so-called “base station”, and asecond transmitting and receiving unit, i.e. a so-called “transponder”.These systems are used for authentication in the motor vehicle field,which represents a main field of application. To achieve a high level ofsecurity of the authentication, the distance or range over which thecommunication can take place is limited to just a few meters for aso-called “passive entry” system, i.e. for unlocking the vehicle doorswhereupon the vehicle door can be opened by pulling the door handle.Furthermore, methods are being developed to prevent an unauthorizedauthentication, especially by means of a redirection or relaying of thecommunication, and for terminating or interrupting the authenticationprocess in the event such a redirection or relaying is detected.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the invention to provide amethod for reliably recognizing and detecting the redirection orrelaying of the communication signals of a contactless datatransmission, for example being used for an authentication. A furtherobject is to increase the difficulty of mimicking the data transmissioncharacteristics and thereby carrying out an undetectable redirection orrelaying of the data transmission. The invention further aims to avoidor overcome the disadvantages of the prior art, and to achieveadditional advantages, as apparent from the present specification. Theattainment of these objects is, however, not a required limitation ofthe present invention.

The above objects have been achieved according to the invention in amethod for detecting a redirection or relaying of a contactless datatransmission between a first transmitting and receiving unit and secondunit that is at least embodied as a transmitting unit. The firsttransmitting and receiving unit includes at least two spatiallyseparated antennas. In the method, an electromagnetic signal istransmitted as an electromagnetic field sequentially from the firstantenna and then from the second antenna of the first transmitting andreceiving unit. The electromagnetic field respectively transmitted bythe first antenna and the second antenna of the first transmitting andreceiving unit is received by the second unit. The respective actualfield strength value of the respective received electromagnetic fieldthat was transmitted by the respective first and second antennas is thendetermined and compared with prescribed nominal field strength valuesstored in a value field. If the actual received and measured fieldstrength values are outside of the value range determined by the nominalvalue field, then a redirection or relaying of the transmission isdetermined to have occurred.

Throughout this specification, the terms “redirection” and “relaying”are used interchangeably to generally refer to any redirection,relaying, retransmission, repeating, or range-extension of thetransmitted data communication, for example via an intermediate devicesuch as a transceiver or radio amplifier relay station, contrary to adirect transmission between the first transmitting and receiving unitand the second (transmitting and) receiving unit without any interveningreceiving and/or transmitting device.

The basic underlying point of the invention is to make it considerablymore difficult to carry out a redirection or relaying of a contactlessdata transmission by comparing measured actual values of a receivedfield strength with prescribed nominal field strength values. This isachieved, as mentioned above, by measuring or determining the fieldstrength value of each respective electromagnetic field received by thesecond unit successively from the respective antennas of the firsttransmitting and receiving unit, and then comparing the determined fieldstrength values of the respective antenna with respective prescribednominal values of a value field. A redirection or relaying of the datacommunication is recognized if the measured actual field strength valuesunacceptably deviate from the range of values defined by the value field(e.g. the actual values do not match or lie within an acceptable rangeof any of the nominal values stored in the value field).

A major advantage of the inventive method is that the transmissioncharacteristics of two spatially separated antennas can only be copiedor imitated by a redirection or relaying device with great effort,expense and difficulty. Thus, a redirection or relaying of thetransmission can be detected with high reliability, because anydeviation from the true transmission characteristics of the originaltransmitting and receiving unit (particularly the true field strengthdistribution of fields radiated from two or more antennas intwo-dimensional or three-dimensional space around the first transmittingand receiving unit) can be detected. In this context, it becomesevermore difficult for the relaying device to imitate the transmissioncharacteristics, the further apart the two or more antennas arespatially separated from each other, and the more the electromagneticfield transmitted from the respective antenna deviates from a perfectspherical signal radiation form. Furthermore, the imitation of thetransmission characteristics by the relaying device is additionally mademore difficult if more than two spatially separated transmissionantennas are used by the transmitting and receiving unit.

In order to avoid a simple superimposing of the respectiveelectromagnetic fields of the respective transmitting antennas of thefirst transmitting and receiving unit, the respective transmittingantennas are driven successively in a sequential manner for respectivelytransmitting the electromagnetic field. In this manner, the secondreceiving unit may sequentially measure or determine the field strengthof the electromagnetic field respectively transmitted from eachrespective one of the antennas of the first transmitting and receivingunit.

In order to quickly and easily carry out the comparison of the fieldstrength values determined by the receiving antenna, it is advantageousto have previously determined or “mapped out” the true electromagneticfield distribution around the respective transmitting antenna for theparticular transmitting frequency, dependent on the distance andlocation relative to the respective antenna. This procedure provides arespective value set (e.g. a value pair) that includes a true fieldstrength of a field of the first antenna and a true field strength of afield of the second antenna at each respective spatial point amongplural spatial points distributed throughout the multi-dimensional spacearound the transmitting and receiving unit. Each value set may furthercomprise a respective reception angle value of the angle formed betweenthe two antennas from the spatial point to which this respective valueset pertains. The resulting plural value sets are stored to form thereofa value field defining a map of the true field strength distribution andreception angles in space around the transmitting and receiving unit. Inthis con-text, the dimensionality of the value field is dependent on thenumber of transmitting antennas being used, and is at least twodimensional in the case of using two transmitting antennas.

Moreover, by prescribing minimum and maximum acceptable field strengthvalues within a value field, a permissible value or communication rangecan be prescribed. In other words, if the actual field strength valueslater fall outside of the range defined by the minimum and maximum fieldstrength values, then a communication will not be permitted.Furthermore, the functional reliability of the method can be increased,especially in connection with a fluctuation of the dampingcharacteristics, by determining and providing a scattering or variationrange around each respective field strength value of the value field.This acceptable range of variation for each field strength value can beselected to form a continuous-coverage map or value field without valuegaps between the respective discrete measurements at the finite numberof respective discrete spatial points. It is further advantageous thatthe inventive method is able to detect a redirection or relaying of theelectromagnetic field independent of the transmitting frequency, that isto say both in the near or close range field as well as in the far orlong range field of the transmitting antenna.

In a further detailed embodiment of the inventive method, the fieldstrength determination of the received electromagnetic field is carriedout in the second receiving unit by means of three antennas, which arerespectively arranged with their effective axes oriented orthogonallyrelative to each other. In order to simultaneously determine theindividual components of the field strength, that is to say theso-called field strength vector, it is advantageous when each receivingantenna is connected individually to a respective single associatedinput channel in which the respective component of the field strengthvector is determined. In this manner it is possible to suppress thecoupling between the individual components, which is caused by amovement or position variation of the antennas during the determination,in connection with a sequential determination of the individualcomponents of the field strength vector. Moreover, with threeorthogonally oriented antennas, a space-saving structural configurationcan be achieved in that two of the orthogonally oriented antennas arearranged in the plane of the third antenna. This can be achievedespecially in the near field of the electromagnetic field, by means oftwo rod-shaped antennas that are arranged in the plane of the thirdannular ring-shaped antenna. In other words, the three antennas togetherform a physical structure with the shape of a cross inside an annularring.

Investigations by the applicants have shown that it is especiallyadvantageous to detect the redirection or relaying of theelectromagnetic signal by determining the magnitude or absolute value ofthe received field strength from the individual components of the fieldstrength vector respectively for the field transmitted from eachrespective antenna, and then comparing the value pairs of the absolutemagnitudes with the value pairs stored in the value field.

It is further advantageous to determine, from the various field strengthvectors, the actual reception angle formed between the two transmittingantennas relative to the receiving unit, and then to compare thisdetermined reception angle with the prescribed or true angle valuesstored in the value field as mentioned above. In this context, eachangular value is, for example, linked or coupled with at least one valuepair of the field strength vectors. Insofar as the determined receptionangle lies within a prescribed interval or range about the prescribedangle value of a respective value set (or pair) in the value field, thenthe possibility of a redirection or relaying of the transmission isexcluded. On the other hand if the determined reception angle fallsoutside of the allowable interval or range about the nominal angle valueof the value set (or pair) stored in the value field, then theoccurrence of a redirection or relaying is detected and indicated. Bycombining the comparison of the actual values of the field strengthvectors, or especially the absolute magnitudes thereof, relative to therespective nominal values therefor stored in the value field, and thecomparison of the actual determined value of the reception anglerelative to the angle values stored in the value field, the reliabilityof the method is increased.

According to another detailed embodiment of the invention it isadvantageous that the results of the value comparisons are stored, sothat the stored values can thereafter be used, for example, to determinethe scattering or variation ranges of the field strength determination.Particularly, by means of the stored values, it is possible to obtainstatistical information regarding a long term or slowly occurringvariation of the transmitting and/or receiving characteristics. Inconnection with the inventive method being carried out in a device in amotor vehicle, the stored information can be read-out at regularintervals, for example, during the normal service intervals of the motorvehicle.

In a further developed embodiment of the invention the magnetic portionof the field strength is determined for a communication in theelectromagnetic near field. It is particularly advantageous that thereception of the signal can be carried out by means of especiallyspace-spacing inductive antennas. In this context, it must be consideredthat the magnetic field of transmitting antennas arranged internally ina motor vehicle is strongly deformed in the exterior space outside ofthe vehicle, due to the effects of the sheet metal body of the vehicle,especially for low frequency transmission signals in the range of 125kHz. Therefore, a copying or imitation of the transmissioncharacteristics of such antennas cannot be achieved with an acceptableor practical effort and expense, so that a redirection or relaying ofthe signal can be detected with great reliability.

It is further advantageous to carry out the value comparison at orbefore the beginning of the data transmission, and to then begin theactual data transmission when no redirection or relaying of the signalhas been detected. It is further advantageous to carry out the valuecomparison repeatedly during the data transmission, so as to therebyincrease the security of long term data transmission. Investigations bythe applicants have shown that it is advantageous especially inconnection with relatively large data quantities, to carry out thedetermination of the field strength on modulated data-carryingelectromagnetic fields. In this context, the electromagnetic field ismodulated, preferably by a phase or frequency modulation, and thedetermination of the received field strength and the comparison of thefield strength values is, for example, carried out during anidentification routine that is based on the modulated electromagneticfield. Insofar as or whenever no redirection or relaying of the signalis detected, the data received and evaluated during the field strengthcomparison are deemed valid. On the other hand, whenever a redirectionor relaying is detected, the data received during the associated fieldstrength comparison are deemed invalid and lead to a failure of theidentification process.

In yet another embodiment of the invention, especially in connectionwith a unidirectional data transmission, it is advantageous to carry outthe value comparison within the receiver. In this regard, the nominalvalue field is stored in a memory unit in the receiver itself. In thismanner, it is not necessary to transmit the received field strengthvalues back to the first transmitting and receiving unit, so thatthereby the total communication time is reduced. On the other hand,another embodiment of the inventive method relates especially to abi-directional data transmission, in which it is advantageous that thefield strength values and/or the result of the comparison of the fieldstrength values are transmitted by the second unit (embodied as areceiving and transmitting unit) back to the first transmitting andreceiving unit. The nominal value field may be stored in the first unit.In this manner, the determination and/or the comparison of the receivedfield strength values can be carried out in the first transmitting andreceiving unit, rather than in the second unit, whereby the circuitcomplexity and the power consumption of the second unit are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now bedescribed in connection with example embodiments thereof, with referenceto the accompanying drawings, wherein:

FIG. 1 is a schematic block diagram of an example embodiment of anarrangement for carrying out the inventive method, with a firsttransmitting and receiving unit having two spatially separated antennas,and a second receiving unit; and

FIG. 2 is a schematic diagram of another example embodiment of anarrangement for carrying out the inventive method, including a firsttransmitting and receiving unit having three spatially separatedantennas arranged in a motor vehicle, and a second receiving unit havingan especially compact arrangement of three receiving antennas.

DETAILED DESCRIPTION OF A PREFERRED EXAMPLE EMBODIMENT AND OF THE BESTMODE OF THE INVENTION

Generally as shown schematically in FIG. 1, an exemplary arrangementaccording to the invention, for carrying out the inventive method,includes a first unit SE1 embodied as a first transmitting and receivingunit SE1, and second unit SE2 embodied as a second receiving unit SE2.The purpose or function of this arrangement is to carry out a dataexchange or communication by transmitting a data-carrying modulatedelectromagnetic field between the first transmitting and receiving unitSE1 and the second receiving unit SE2, and during this datacommunication, to detect any redirection or relaying of theelectromagnetic field (e.g. via a relay transceiver rather than directlyfrom the first unit SE1 to the second unit SE2) by comparing actualreceived field strength values determined in the second receiving unitSE2 with prescribed nominal field strength values.

Next, the physical components or structural features of the illustratedarrangement will be described in further detail. The first transmittingand receiving unit SE1 comprises a control unit CU, which isrespectively connected to a first output driver OD1 and to a secondoutput driver OD2. A first transmitting antenna SA1 is connected withthe first output driver OD1, and a second transmitting antenna SA2 isconnected with the second output driver OD2.

The second receiving unit SE2 comprises three antennas EA1, EA2 and EA3that are respectively oriented orthogonally relative to each other. Arespective individual input channel is allocated and connected to eachrespective one of the receiving antennas EA1, EA2 and EA3, in that therespective receiving antennas EA1, EA2 and EA3 are respectivelyconnected by a respective signal amplifier V1, V2 and V3 to an inputEAD1, EAD2 and EAD3 of an A/D converter AD1. Furthermore, each inputEAD1, EAD2 and EAD3 of the A/D converter AD1 is respectively connectedwith a respective associated input ED1, ED2 and ED3 of a demodulatorDEM. In this manner, the signal received by each one of the receivingantennas EA1, EA2 and EA3 can be individually demodulated to obtain andevaluate the data being carried by the modulated signal as received byeach of the receiving antennas. The A/D converter AD1 and thedemodulator DEM are also each respectively connected with a signalprocessor SP, which in turn is further connected with a memory unit MEM.

In the following, the functional operation of the arrangement will beexplained in further detail. The first transmitting and receiving unitSE1 transmits an electromagnetic signal as an electromagnetic fieldE1(SA1) by means of the first transmitting antenna SA1, and then(sequentially thereafter), transmits an electromagnetic field E2(SA2) bymeans of the second transmitting antenna SA2. In this context, therespective electromagnetic fields E1(SA1) and E2(SA2) transmitted by thefirst transmitting antenna SA1 and by the second transmitting antennaSA2 are preferably the same electromagnetic field in succession or witha time delay being transmitted first via the first transmitting antennaSA1 and thereafter via the second transmitting antenna SA2 so as toavoid a simultaneous overlap of the two fields. More generally, theamplitude of the electromagnetic field E1(SA1) transmitted via the firsttransmitting antenna SA1 preferably corresponds to the amplitude of theelectromagnetic field E2(SA2) transmitted via the second transmittingantenna SA2.

Next, by means of the three receiving antennas EA1, EA2 and EA3 of thesecond receiving unit SE2, the respective three orthogonal components ofthe field strength of the field E1(SA1) received from the firsttransmitting antenna SA1 are determined. Namely, the three individualcomponents of the field strength vector E1 are processed individually inthe three respective input channels in that they are respectivelyindividually converted into digital signals in the A/D converter AD1 andthe corresponding digital signals are provided to the signal processorSP. Thereafter, in a similar manner, the three respective components ofthe field strength vector E2 of the electromagnetic field E2(SA2) asreceived by the three receiving antennas EA1, EA2 and EA3 from thesecond transmitting antenna SA2 are digitized through the A/D converterAD1 and provided to and processed in the signal processor SP.Particularly, the signal processor SP compares the two magnitude valuesof the field strength vectors E1 and E2 with the respective value pairsthat have been stored in a value field in the memory unit MEM.Furthermore, the signal processor SP calculates a reception angle W1from the two field strength vectors E1 and E2 and compares thisreception angle W1 with respective nominal values for the receptionangle stored in the memory unit MEM. This reception angle W1 representsthe angle between the two transmitting antennas SA1 and SA2 from theangle vertex or reference point of the second receiving unit SE2.

If both the field strength values as well as the value of the receptionangle W1 correspond with the respective appropriate values of a givenvalue pair or value set prescribed by, i.e. stored in, the memory unitMEM, then it is assumed that no redirection or relaying of theelectromagnetic signal has taken place, i.e. no redirection or relayingis detected. As a result, the data that have been demodulated andobtained by the demodulator DEM are accepted as valid and are furtherprocessed in the signal processor SP. The signal processor SP, as wellas the other circuit components, may comprise any combination ofconventionally known hardware and software for carrying out thefunctions and process steps as described herein. The processing of thereceived data in the signal processor SP may, for example, involve anauthentication or identification process to authenticate and authorizecarrying out and acting on the data communication.

An advantage of the inventive method is that an unauthorized extendingof the communication path or link via a relay transceiver or the likecan be detected in a reliable manner through determining the respectivefield strength vectors by obtaining the field strength of theelectromagnetic fields respectively transmitted by the two or moretransmitting antennas. Furthermore, the minimum and maximum allowabledistance for a valid communication between the first transmitting andreceiving unit SE1 and the second receiving unit SE2 can be prescribedby imposing maximum and minimum field strength values on the value pairsstored in the value field in the memory unit MEM. The security against aredirection or relaying can be further increased by repeatedlydetermining the field strength during a communication period, and/or byutilizing more than two transmitting antennas for the first transmittingand receiving unit SE1.

A further advantage of the inventive method is that a simplelocation-dependent determination of the true field strength distributionof the electromagnetic field about the respective transmitting antennamakes it possible to determine the value field for a given transmittingand receiving arrangement. Namely, the corresponding value tuple orrelated value set as well as the reception angle are respectively storedin the value field respectively for plural points in space about thetransmitting and receiving unit. The number of the points in space isselected for the desired degree of precision. In order to increase thereliability of the comparison of the actual measured values with theprescribed nominal values, the respective value pairs and the receptionangle values are each associated with an acceptable scattering orvariation range. In this context, the magnitude of the scattering orvariation range depends on the deformation of the field distribution,and on the system transmission scattering characteristics of thetransmitting and receiving arrangement, and on the spatial precision,i.e. the number of spatial points and associated value tuples that havebeen determined and stored in the value field.

It is impossible to copy or imitate the overall transmissioncharacteristic using a relaying device with an acceptable or practicaleffort and expense, even if the spacing distance between thetransmitting antennas lies within the allowable communication range ordistance, for example in an application in a motor vehicle. Furthermore,the inventive method may be carried out in connection with both aunidirectional communication as well as a bi-directional communication.Especially in a bi-directional communication between the two units SE1and SE2, the determination of the field strength vectors can be carriedout in the first transmitting and receiving unit SE1, in that theindividual components of the respective received field strength vectorare transmitted back from the second unit SE2 to the first transmittingand receiving unit SE1. In this context, of course, the second unit SE2must also be embodied as a transmitting and receiving unit having bothreceiving and transmitting capabilities. In this manner, the circuitcomplexity as well as the power consumption of the second transmittingand receiving unit can be considerably reduced, because the evaluationand processing of the received field strength information does not needto be carried out directly in the second unit. Furthermore, since thedetermination and the comparison of the field strength values can becarried out directly during the data evaluation within an authenticationprocess, the total communication time is only slightly increased.

FIG. 2 schematically shows a second example embodiment in which thefirst transmitting and receiving unit (not shown in detail) is embodiedas a base station BS that is installed in a motor vehicle CA. The basestation BS is respectively connected to three transmitting and receivingantennas AN1, AN2 and AN3 arranged spatially separated from each otherin or on the motor vehicle CA. The second transmitting and receivingunit (not shown in detail) is embodied as a transponder TR that isinstalled or integrated in a key fob or key KE, for example a key forunlocking and operating the motor vehicle CA. The transponder TRcomprises or is connected to three transmitting and receiving antennasEN1, EN2 and EN3. The antenna EN3 is embodied as an annular ring-shapedantenna EN3. The two antennas EN1 and EN2 are embodied as rod-shapedantennas EN1 and EN2 arranged preferably crossing one another in theplane of the annular ring-shaped antenna EN3. In this manner, theeffective axes of these three antennas EN1, EN2 and EN3 are orientedorthogonally relative to each other, while all three antennas arephysically arranged lying in a single plane.

A bidirectional data transmission is to be carried out between the basestation BS and the transponder TR in the low frequency range at 125 kHzby means of an inductive coupling in the electromagnetic near field. Tocarry out the authentication and data communication process, the basestation BS radiates or transmits the electromagnetic field sequentiallyin succession from the three transmitting and receiving antennas AN1,AN2 and AN3. Since, for an effective communication, the transmitting andreceiving antennas EN1, EN2 and EN3 of the transponder TR are located inthe near field of the transmitting antennas AN1, AN2 and AN3,essentially the magnetic components of the electromagnetic field areeffective. In this context, generally the magnetic field shape or fieldprogression outside of the motor vehicle CA deviates from a sphericalfield form, because it is deformed proportionally with the magnitude ofthe permeability of the body of the motor vehicle CA at any location.

In the transponder TR, by means of the transmitting and receivingantennas EN1, EN2 and EN3, the orthogonal components of the fieldstrength vectors E1, E2 and E3 respectively associated with thetransmitting antennas AN1, AN2, and AN3 are received and determined insequential succession. In this context, beginning with the transmissionof the field by the first antenna AN1 of the base station BS, theassociated three components of the magnetic field strength vector E1 aredetermined, e.g. measured, in the transponder TR and then the individualcomponents are transmitted back from the transponder TR to the basestation BS. Then, this same process is carried out for the respectivecomponents of the field strength vector E2 associated with the secondantenna AN2, and then for the field strength vector E3 associated withthe third antenna AN3.

Preferably, in the context of the transmission of the values of theindividual components back to the base station BS from the transponderTR, the values are preferably modulated by the transponder TR onto areply or response signal as digital data in an encoded form. In the basestation BS, the respective associated magnitudes or absolute values aswell as the reception angle W2 between the vectors EN1 and EN2, and thereception angle W3 between the vectors EN2 and EN3, are determined fromthe components of the respective vectors EN1, EN2 and EN3, and are thencompared with the respective associated values of value sets or tuplesstored in a value field. Furthermore, it is advantageous according tothe invention, that if the determined actual values correspond with thestored nominal values, then the base station BS transmits acorresponding reply or response signal to the transponder TR to informthe transponder TR of the validity of the evaluated data, in order tofurther carry on the authentication process between the base station BSand the transponder TR.

An advantage of the inventive method, in connection with abi-directional data transmission, is that the determination of thevectors and the comparison of the determined actual values with theprescribed nominal values can respectively be carried out in the basestation BS. This allows the complexity of an integrated circuit of thetransponder TR as well as the power consumption of the transponder TR tobe reduced, which is beneficial when the transponder TR is to beembodied in a small key KE or the like.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated that it is intended to coverall modifications and equivalents within the scope of the appendedclaims. It should also be understood that the present disclosureincludes all possible combinations of any individual features recited inany of the appended claims.

1. A method of detecting a relaying of a contactless electromagnetic data transmission, comprising the steps: a) transmitting a first electromagnetic field from a first transmitting antenna of a first unit having at least a transmitting capability; b) sequentially after said step a), transmitting a second electromagnetic field from a second transmitting antenna of said first unit, wherein said first and second transmitting antennas are spatially separated from each other; c) sequentially receiving said first electromagnetic field and said second electromagnetic field with a second unit having at least a receiving capability; d) determining a first actual field strength value of a field strength of said first electromagnetic field and a second actual field strength value of a field strength of said second electromagnetic field as respectively received by said second unit; e) comparing said first and second actual field strength values respectively with prescribed first and second nominal field strength values of a nominal value field; and f) detecting that a relaying of at least one of said first and or second electromagnetic fields has occurred if said comparing determines that said first and second actual field strength values unacceptably deviate from said prescribed first and second nominal field strength values of said nominal value field.
 2. The method according to claim 1, wherein said first and second electromagnetic fields as transmitted respectively by said first and second transmitting antennas both have the same amplitude.
 3. The method according to claim 1, wherein said first and second electromagnetic fields as transmitted respectively by said first and second transmitting antennas both correspond to the same electromagnetic signal transmitted successively from said first transmitting antenna and then again from said second transmitting antenna.
 4. The method according to claim 1, further comprising comparing said first and second actual field strength values with a prescribed maximum value and with a prescribed minimum value, and detecting that a relaying of at least one of said first and or second electromagnetic fields has occurred if at least one of said first and or second actual field strength values lies above said maximum value or below said minimum value.
 5. The method according to claim 1, wherein said comparing of said step e) comprises determining whether said first and second actual field strength values respectively fall within a prescribed acceptable variation range respectively around said first and second nominal field strength values, and if at least one of said first and or second actual field strength values falls outside of said prescribed acceptable variation range respectively around said first and second nominal field strength values then it is determined that said first and second actual field strength values unacceptably deviate from said prescribed first and second nominal field strength values.
 6. The method according to claim 1, wherein said nominal value field includes a plurality of value sets that each respectively include a respective one of said first nominal field strength values and a respective one of said second nominal field strength values which are prescribed for a respective spatial point in multi-dimensional space around said first unit, and wherein said comparing of said step e) comprises determining whether said first and second actual field strength values respectively match or fall acceptably close to said first and second nominal field strength values of any one of said value sets.
 7. The method according to claim 6, further comprising a preliminary step of mapping out a field strength distribution pattern of said first electromagnetic field transmitted by said first transmitting antenna and of said second electromagnetic field transmitted by said second transmitting antenna in said multi-dimensional space around said first unit, by transmitting said first and second electromagnetic fields successively from said first and second transmitting antennas respectively, measuring a respective first true field strength value of said first electromagnetic field and a respective second true field strength value of said second electromagnetic field respectively at each one of plural spatial points in said multi-dimensional space around said first unit, and respectively storing said first and second true field strength values as said first and second nominal field strength values of a respective one of said value sets respectively for each respective one of said spatial points in said multi-dimensional space around said first unit to make up said nominal value field.
 8. The method according to claim 6, wherein each respective one of said value sets respectively further includes a respective nominal reception angle value of an angle between said first and second transmitting antennas with respect to a respective angle vertex at said respective spatial point associated with said respective value set; wherein said method further comprises determining from said first and second electromagnetic fields received by said second unit an actual reception angle value of a reception angle between said first and second transmitting antennas relative to said second unit, and comparing said actual reception angle value with said nominal reception angle values of said value sets; and wherein said step f) comprises detecting that a relaying of at least one of said first and or second electromagnetic signals has occurred if said first and second actual field strength values do not respectively match or fall acceptably close to said first and second nominal field strength values or said actual reception angle value does not match or fall acceptably close to said respective nominal reception angle value of any single one of said value sets.
 9. The method according to claim 1, wherein said step c) comprises respectively receiving three mutually orthogonal field strength components of a first field vector of said first electromagnetic field and of a second field vector of said second electromagnetic field, and said step d) comprises determining said first actual field strength value as an absolute magnitude of said first electromagnetic field from said orthogonal field strength components of said first field vector of said first electromagnetic field and determining said second actual field strength value as an absolute magnitude of said second electromagnetic field from said orthogonal field strength components of said second field vector of said second electromagnetic field.
 10. The method according to claim 9, further comprising determining a reception angle between said first and second transmitting antennas relative to said second unit from said field vectors of said first and second electromagnetic fields.
 11. The method according to claim 1, further comprising transmitting a third electromagnetic field from a third transmitting antenna of said first unit which is spatially separated from said first and second transmitting antennas sequentially after said step b), receiving said third electromagnetic field with said second unit, determining a third actual field strength value of a field strength of said third electromagnetic field as received by said second unit, comparing said third actual field strength value with prescribed third nominal field strength values of said nominal value field, and detecting that a relaying of at least one of said first, second and or third electromagnetic fields has occurred if said comparing determines that at least one of said first, second and or third actual field strength values unacceptably deviates respectively from said prescribed first, second and third nominal field strength values of said nominal value field.
 12. The method according to claim 1, wherein said second unit comprises three receiving antennas having respective effective axes that are oriented orthogonally relative to each other, and wherein said step c) is carried out by said three receiving antennas.
 13. The method according to claim 12, wherein said three receiving antennas comprise an annular ring-shaped antenna extending on a plane and two rod-shaped antennas extending on said plane of said ring-shaped antenna.
 14. The method according to claim 1, wherein said first and second actual field strength values are respective absolute magnitude values of a field strength of said first and second electromagnetic fields respectively.
 15. The method according to claim 1, further comprising determining a reception angle between said first and second transmitting antennas from said actual field strength values, and comparing said reception angle with a prescribed angle to determine whether a correspondence exists therebetween.
 16. The method according to claim 1, further comprising storing results of said comparing of said step e).
 17. The method according to claim 1, wherein said actual field strength values are magnetic field strength values of respective magnetic portions of said first and second electromagnetic fields.
 18. The method according to claim 1, further comprising carrying out a data transmission by modulating data onto at least one of said first and or second electromagnetic signals.
 19. The method according to claim 18, wherein said modulating comprises a phase modulation or a frequency modulation.
 20. The method according to claim 18, wherein said steps a) to e) are carried out before or at a beginning of said data transmission.
 21. The method according to claim 18, wherein said steps d) and e) are carried out repeatedly at intervals while carrying out said data transmission.
 22. The method according to claim 1, wherein said step e) is carried out in said second unit.
 23. The method according to claim 22, wherein said second unit further has a transmitting capability and said first unit further has a receiving capability, and further comprising transmitting a result of said comparing of said step e) from said second unit to said first unit.
 24. The method according to claim 1, wherein said second unit further has a transmitting capability and said first unit further has a receiving capability, and further comprising transmitting said actual field strength values from said second unit to said first unit, and wherein said step e) is carried out in said first unit.
 25. A method comprising: a) receiving a first electromagnetic field and a second electromagnetic field at a receiver; b) determining a first actual field strength value of the first electromagnetic field and a second actual field strength value of the second electromagnetic field; c) comparing the first and second actual field strength values with first and second nominal field strength values, respectively, of a nominal value field; and d) determining whether a relaying of the first or second electromagnetic fields has occurred if the comparing step indicates that one or more of the first and second actual field strength values deviates from the first or second nominal field strength values, respectively, by more than a threshold amount.
 26. The method of claim 25, wherein the first electromagnetic field is transmitted from a first transmitting antenna of a transmitter and the second electromagnetic field is transmitted from a second transmitting antenna of the transmitter.
 27. The method of claim 26, wherein the first and second transmitting antennas are spatially separated from each other.
 28. The method of claim 25, wherein the first and second electromagnetic fields have the same amplitude.
 29. The method of claim 25, wherein the first and second electromagnetic fields include the same signal and wherein the first and second electromagnetic fields are transmitted from different antennas of a transmitter.
 30. A apparatus comprising: a receiver, configured to receive a first electromagnetic field and a second electromagnetic field, configure to determine a first actual field strength value of the first electromagnetic field and a second actual field strength value of the second electromagnetic field, configured to compare the first and second actual field strength values with first and second nominal field strength values, respectively, of a nominal value field, and configured to determine a relaying of at least one of the first or second electromagnetic fields has occurred if the comparison indicates that at least one of the first or second actual field strength values deviates from the first or second nominal field strength values, respectively, by more than a threshold amount.
 31. The apparatus of claim 30, further comprising: a transmitter, configured to transmit the first electromagnetic field from a first antenna, and configured to transmit the second electromagnetic field from a second antenna.
 32. The apparatus of claim 31, wherein the first and second electromagnetic fields include the same signal.
 33. The apparatus of claim 30, wherein the receiver comprises three antennas orthogonally oriented. 